Ask Tom: How unusual is a full moon on Christmas Day?

If the weather cooperates, Chicagoans will be treated to a rare Christmas morning full moon at 5:11 a.m., the first occurrence of a full moon on Christmas in 38 years. The Christmas full moon in 1977 was one of just four in the 20th century, preceded by others in 1901, 1920 and 1950.

Dan Joyce, an astronomer at Triton College's Cernan Earth and Space Center, tells us that the average frequency of any lunar phase occurring on a particular date is twice in any 59-year period. This century will feature five. Following this year's, Christmas full moons will return in 2034, 2053 and 2072 and 2091.